To detect audio signals, MEMS microphones typically have a static backplate that supports and forms a capacitor with a flexible diaphragm. Audio signals cause the diaphragm to vibrate, thus producing a changing capacitance. Circuitry receives and converts this changing capacitance into electrical signals that can be further processed.
To sense an incoming audio signal, the diaphragm should be able to vibrate in a substantially unimpeded manner. If the backplate were solid, then air between it and the diaphragm would significantly resist that vibration. Accordingly, MEMS microphones typically have a plurality of generally round holes extending through the backplate. Air in the space between the diaphragm and backplate therefore can escape through these through-holes, thus providing reasonable sensitivity to incoming audio signals.
Round through-holes typically provide excellent air resistance properties—compared to other shapes with the same area, they often create the lowest air resistance. Their geometry, however, undesirably limits their total number through the backplate.